Seasonal Differences in Size Spectra

In plotting seasonal spectra exponents it was apparent that in the three southern areas there were large differences in size spectra between the spring and fall.

This doc is a quick exploration into what might be happening there in terms of community changes or recruitment.

Size Structure Trends

One thing worth looking at is the abundance of different size classes.

If we break the size distribution into bins we can see how relative abundances have fluctuated.

My interests here are to assess whether seasonal “steepening” is a result of bottom-up abundance gains in smaller size classes, or top-down loss/absence of larger individuals.

The next code chunk is for assigning log10 bin labels on which we can summarize abundance + biomass. This first step builds the labels as a dataframe for a specified size range.

This next one adds those labels to a dataframe using a specified log size increment.

If we run the assignment we get a dataframe that can be grouped on:

Next we can aggregate years and seasons to get a sense of where regional abundance and biomass is distributed across the size bins.

The next panel plots changes in normalized abundance. This is the aggregate abundance divided by the size-bin width.

Size Bin Changes in Time

Normalized Biomass Changes

This panel are the same, but for normalized biomass.

Seasonal Differences in Size Bins

The intra-annual variability in spectra slopes for the Mid-Atlantic Bight in particular are really throwing me off.

The following figure shows what the within-year changes in normalized biomass is for each size bin.

What happened in 2017?

Sampling in the two southern regions not conducted during 2017.

Species Specific Size-Bin Seasonality

Really curious if there are any key species that are driving things or that stand out:

Species standing out seasonally for normalized abundance:

Silver Hake
Butterfish

Species standing out for normalized biomass:

spiny dogfish
Spotted hake

General notes, GOM seems way more seasonal (more species showing up in plots, and tendency to lose biomass in the fall)

Seasonal Change in Size Spectra

Seasonal Difference in Median Size

Are the same-year median size differences plausible for growth? They could just have grown.

Mid-Atlantic Bight Dogfish

There is some evidence above that larger individuals in MAB-Spring are driving the trend there, try digging into that a little.

Highly Seasonal Species